A solid polymer fuel cell is supplied with hydrogen and oxygen, to generate power, and the power generation reaction produces water. As a material of construction of a gas diffusion layer constituting membrane-electrode assemblies that cause power generation reaction of a fuel cell, a carbon paper comprising carbon fibers bound by a carbonized resin is generally used. For applications requiring high power densities such as motor vehicles, the fuel cell is operated in a high current density range. So, the amount of water produced per unit reaction area also increases. Therefore, in such cases, how efficiently the water produced by the reaction should be discharged is important, and the carbon paper used as the material of construction of the gas diffusion layer of fuel cell is required to have a high gas permeability.
To address the above-mentioned problem, in Patent Document 1, a precursor fiber sheet decreased in the amount of a thermosetting resin to be impregnated into a carbon fiber paper is treated with heating and pressurizing at an appropriate pressure. In this production method, while the density of carbon fibers is maintained, the amount of the carbon used as a binder is decreased to lower the density of the carbon paper, for thereby enhancing the gas permeability in the thickness direction. However, if the amount of the carbon used as a binder is decreased, the binding points between the carbon fibers and the carbonized resin decrease to raise a problem that the resistivity of the carbon paper in the thickness direction increases. Therefore, in the carbon paper described in Patent Document 1, there is a tradeoff relation between the gas permeability in the thickness direction and the resistivity in the thickness direction, and it is difficult to satisfy both sufficiently.
On the other hand, in Patent Document 2, a precursor fiber sheet obtained by impregnating a carbon fiber paper sheet having a low unit area weight with graphite particles and a thermosetting resin is merely treated with heating instead of being treated with heating and pressurizing. Since a carbon paper obtained by the production method is higher in the density of highly conductive graphite particles though lower in the density of carbon fibers, the carbon paper has both low electric resistivity and high air permeability in the thickness direction. However, since the mechanical strength of the carbon paper greatly depends on the density of the carbon fibers contained in the carbon paper, the carbon paper described in Patent Document 2 has a problem that the residual deformation after compression in the thickness direction is large.
If a carbon paper having a large residual deformation after compression in the thickness direction is stacked in a fuel cell, it can happen that the carbon paper drops into grooves formed in a flow field plate and blocks gas passages and that the carbon paper diminishes in thickness, with lapse of time and reduces the conduction with the flow field plate. These can cause the fuel cell to decline in performance.
Further, in the carbon paper production method described in Patent Document 2 in which the precursor fiber sheet is not treated with heating and pressurization, the thickness of the obtained carbon paper is decided by any chance in the course of the mere heat treatment, depending on the composition and the unit area weight of the precursor fiber sheet. Therefore, it is difficult to appropriately control the thickness of the obtained carbon paper. Furthermore, the production method is not preferred either since the fluctuations in the composition and the unit area weight of the precursor fiber sheet directly lower the thickness accuracy of the carbon paper.
As described above, it is difficult for the prior art to provide carbon paper having all the properties of high gas permeability, high conductivity and small residual deformation after compression, which are required as a carbon paper used as a gas diffusion layer of a fuel cell.
The object of the invention is to provide a carbon paper having properties required as a carbon paper used as a gas diffusion layer of a fuel cell, particularly all of high gas permeability, high conductivity and small residual deformation after compression, though it is difficult for the prior art to provide such a carbon paper having all of the properties.    Patent Document 1: JP 9-157052 A    Patent Document 2: JP 2004-31326 A